Blender shrinkwrap modifier
CG Masters explained their modelling techniques much more thoroughly in their Hard Surface Modelling course series. I'll try to explain it here.

They are using the Shrinkwrap Modifier to avoid shading issues. When you are using many modifiers such as Boolean and Subdivide Surface modifiers, it is common to encounter strange/unwanted shading (such as creases and sharp edges) on the surfaces.

Thus, to avoid those shading issues, they create two meshes, a base/guide mesh and a detailed copy mesh. The base mesh has a minimal amount of detail on it, its purpose is to provide the proper shape and shading to the detailed mesh. The copy mesh has all of the details, such as boolean cutouts, on it. Since this results in shading issues, they use the Shrinkwrap Modifier to stick the Copy mesh onto the Base Mesh. Doing so transfers all of the normals from the base to the copy, fixing the shading while keeping the detail.